In recent years, oil and natural gas resources located on land or in so-called shallow seas having a water depth of up to approximately 500 meters have been drying up, so sea bottom oil fields in so-called deep seas at 1000-3000 meters below the ocean surface, for example, are being actively developed. With deep sea oil fields, it is necessary to transport crude oil or natural gas from the wellhead of an oil well or natural gas well located on the sea bottom to a platform on the surface of the sea using steel pipes referred to as flow lines and risers.
A high internal fluid pressure due to the pressure of deep underground layers is applied to the interior of steel pipes constituting flow lines installed in deep seas. In addition, when operation is stopped, they are subjected to the water pressure of deep seas. Steel pipes constituting risers are also subjected to repeated strains due to waves.
Flow lines used herein are steel pipes for transport which are installed along the contours on the ground or the sea bottom, and risers are steel pipes for transport which rise from the surface of the sea bottom to platforms on the surface of the sea. When such pipes are used in deep sea oil fields, it is considered necessary for their thickness to normally be at least 30 mm, and in actual practice, it is customary to use thick-walled pipes having a thickness of 40-50 mm. It can be seen from this fact that these materials are used in severe conditions.
FIG. 1 is an explanatory view schematically showing an example of an arrangement of risers and flow lines in the sea. In this figure, a wellhead 12 provided on the sea bottom 10 and a platform 14 provided on the water surface 13 immediately above it are connected by a top tension riser 16. A flow line 18 installed on the sea bottom extends from an unillustrated remote wellhead to the vicinity of the platform 14. The end portion of this flow line 18 is connected to the platform 14 by a steel catenary riser 20 which extends upwards in the vicinity of the platform.
The environment of use of the illustrated risers and flow line is severe, and is said to reach a temperature of 177° C. and an internal pressure of 1400 atmospheres. Accordingly, steel pipes used for risers and flow lines must be able to withstand such a severe environment of use. Moreover, a riser is subjected to bending stress due to waves, so it must also be able to withstand such external influences.
Accordingly, steel pipes having a high strength and high toughness are desired for risers and flow lines. In addition, in order to ensure high reliability, seamless steel pipes are used instead of welded steel pipes. For welded steel pipes, techniques for manufacturing steel pipes having a strength exceeding X80 grade have already been disclosed. For example, Patent Document 1 (JP H09-41074 A1) discloses a steel which exceeds X100 grade (a yield strength of at least 689 MPa) specified in API standards. A welded steel pipe is formed by first manufacturing a steel plate, forming the steel plate into a tubular shape, and welding it to form a steel pipe. In order to impart important properties such as strength and toughness when manufacturing a steel plate, the microstructure is controlled by applying thermomechanical heat treatment to the steel plate during rolling thereof. Patent Document 1 also carries out thermomechanical heat treatment, when a steel plate is being hot rolled, such that its microstructure is controlled so as to contain strain-induced ferrite, thereby achieve the properties of the steel pipe after welding. Accordingly, the technique disclosed in Patent Document 1 can only be realized by a rolling process for a steel plate to which thermomechanical heat treatment can easily be applied by controlled rolling. Therefore, this technique can be applied to a welded steel pipe but not to a seamless steel pipe.
As long as seamless steel pipes are concerned, in recent years, seamless steel pipes of X80 grade have been developed. It is difficult to apply to seamless steel pipes the above-described technique utilizing thermomechanical heat treatment which was developed for welded steel pipes, so basically it is necessary to obtain desired properties by heat treatment after pipe formation. A technique for manufacturing a seamless steel pipe of X80 grade (a yield strength of at least 551 MPa) is disclosed in Patent Document 2 (JP 2001-288532 A1), for example. However, as disclosed in the examples of Patent Document 2, the technique in that document is validated only with a thin-walled seamless steel pipe (wall thickness of 11.1 mm) which essentially has good hardenability by quenching. Therefore, even if the technique disclosed therein is employed, when manufacturing a thick-walled seamless steel pipe (wall thickness of around 40-50 mm) actually used for risers and flow lines, the cooling rate at the time of quenching of the pipe becomes slow, particularly at the central portion thereof due to its thickness, and there is the problem that a sufficient strength and toughness cannot be obtained. This is because the cooling rate is slow, and with a conventional alloy design, it is difficult to obtain a uniform microstructure and there is a high probability of a brittle phase developing.